Journal of Vision (2007) 7(13):14, 1–10

http://journalofvision.org/7/13/14/

1

How many objects can you track?: Evidence for a resource-limited attentive tracking mechanism
Department of Brain and Cognitive Sciences
Massachusetts Institute of Technology,
Cambridge, MA, USA

George A. Alvarez
Steven L. Franconeri

Department of Psychology Northwestern University,
Evanston, IL, USA

Much of our interaction with the visual world requires us to isolate some currently important objects from other less important objects. This task becomes more difficult when objects move, or when our field of view moves relative to the world, requiring us to track these objects over space and time. Previous experiments have shown that observers can track a maximum of about 4 moving objects. A natural explanation for this capacity limit is that the visual system is architecturally limited to handling a fixed number of objects at once, a so-called magical number 4 on visual attention. In contrast to this view, Experiment 1 shows that tracking capacity is not fixed. At slow speeds it is possible to track up to 8 objects, and yet there are fast speeds at which only a single object can be tracked. Experiment 2 suggests that that the limit on tracking is related to the spatial resolution of attention. These findings suggest that the number of objects that can be tracked is primarily set by a flexibly allocated resource, which has important implications for the mechanisms of object tracking and for the relationship between object tracking and other cognitive processes.
Keywords: multiple object tracking, MOT, multi-element tracking, attentive tracking, FINST, FLEX, attention, capacity
Citation: Alvarez, G. A., & Franconeri, S. L. (2007). How many objects can you track?: Evidence for a resource-limited attentive tracking mechanism. Journal of Vision, 7(13):14, 1–10, http://journalofvision.org/7/13/14/, doi:10.1167/7.13.14.

Introduction
Tracking moving objects over space and time is a fundamental part of making sense of a dynamic visual world. Whether driving on a busy highway, playing team sports, or watching one’s children at the playground, one often maintains attention on multiple moving objects simultaneously. To explore this ability in the laboratory, researchers have employed the multiple object tracking task (Pylyshyn & Storm, 1988). Typically, a set of identical items is presented and a subset of target items is cued, then all items move randomly about the screen for several seconds. During this time, all of the items appear identical and the eyes can only fixate directly on one target at a time. Thus, to track multiple targets concurrently, observers are required to “mentally track” the target items as they move about the display. At the end of the trial, all of the items stop and the observer must indicate which items were the original targets.
Studies employing this task have been used to investigate a wide range of topics in visual cognition, including determining what counts as an object for object-based attention (Scholl & Pylyshyn, 1999; Scholl, Pylyshyn, &
Feldman, 2001), the dynamics of attention in depth
(Viswanathan & Mingolla, 2002), the coordinate systems underlying attention (Liu et al., 2005), the limits on divided or multifocal attention (Alvarez, Horowitz, doi: 1 0. 11 67 / 7 . 1 3 . 1 4

Aresenio, DiMase, & Wolfe, 2005; Cavanagh & Alvarez,
2005), age differences in attention (Trick, Audet, &
Dales, 2003), and deficits in attention for different patient populations (Ho et al., 2006; O’Hearn, Landau, &
Hoffman, 2005).
Given the broad range of work that employs the multiple object-tracking task, it is important to understand the nature of limits on tracking at a basic level. In the current paper, we investigate whether the limit on the number of objects that can be tracked is fixed (the fixedarchitecture model), or whether the limit on tracking is set by a resource that can be flexibly allocated to objects depending on the demands of